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JayDee
United Kingdom
13 Posts |
 Posted - 30 Jul 2017 : 17:31:54
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Hi guys,
Looking for a bit of advice, I'm laying out a short USB (full speed) connection.
The track lengths are short (under 20mm) and go from the Microcontroller to a USB micro connector. 2 layer board and solid ground plane on bottom side.
Using the Easy-PC design calculator to workout the best widths and spacing for the differential micro strip.
I'm using the following values, 1.6mm FR4, Relative Permittivity 4.5.
From a bit of internet reading I think I'm aiming for around 50ohm characteristic impedance and 90 Ohm differential impedance.
But this is taking me towards a 2.5mm track and 1.6mm spacing between them....which just seems too big overall.
Does this seem right and what general USB layout values do you guys use?
Thanks, John. |
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Iain Wilkie
United Kingdom
1019 Posts |
Posted - 30 Jul 2017 : 19:11:04
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You could shrink back a bit .... normal track spacing these days is 10 thou that would put the track width at about 50 thou on 1.6mm FR4 substrate.
Iain |
Edited by - Iain Wilkie on 30 Jul 2017 19:29:21 |
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JayDee
United Kingdom
13 Posts |
Posted - 31 Jul 2017 : 11:06:58
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Thanks Iain,
Using the calculator for differential microstrip
Using 1.6mm thickness, 1oz Copper,1.27mm (50thou) tracks, 0.25mm(10thou) spacing and Er of 4.5
gives a characteristic impedance 79Ohm, Differential Impedance 92.7 Ohm.
I'll give these a go and see how we get on with the prototype board.
As ever, everything decision is a balance, My current problem with laying USB tracks is knowing which aspects to be strict on and which have some wiggle room.
Thanks for the input..
John.
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Iain Wilkie
United Kingdom
1019 Posts |
Posted - 31 Jul 2017 : 12:15:49
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Aim for differential of 90R that's the important one. Taking into consideration of manufacturing tolerances etc nothing needs to be bob-on.
Aim to get the lengths as short as possible and try and get the lengths equal to within a few 10's of thou. Try to avoid vias.
Iain |
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Scazon
United Kingdom
67 Posts |
Posted - 31 Jul 2017 : 14:24:10
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| USB full speed is only 12Mb/ second. I've never had any problems with it (in a few dozen designs, mostly using FTDI or STM32) even with 6 thou tracks and spacing, whatever impedance that pans out to- though I generally keep the tracks short anyway. Just emulate the nurse, slap it on and take no notice. High speed is another bowl of porridge altogether. |
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Iain Wilkie
United Kingdom
1019 Posts |
Posted - 31 Jul 2017 : 15:44:47
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Yup it'll work, but watch out for EMC, get that wrong and it will hurt badly !
iain |
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JayDee
United Kingdom
13 Posts |
Posted - 31 Jul 2017 : 17:08:30
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| Yeah :) thanks guys. I've committed something to copper which should have a decent 90ohm impedance and matched trace lengths .. I'll see how we go when it come back from Manufacture. :) |
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Topic |
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USB Layout - Differential Tracks